Abstract Despite overwhelming evidence that the immune system plays a significant role in amyotrophic lateral sclerosis (ALS) pathogenesis, information about the role of the immune system in human patients is lacking. Our preliminary data show that natural killer (NK) cells are upregulated in the blood of ALS patients. NK cells function as the innate immune analogs of adaptive CD8 T cells. These cells can also skew immune polarization or directly lyse infected and/or dying cells (NK) without the aid of antigen presenting cells. NK cell numbers are increased in the spinal cord of ALS mice, and other innate lymphoid cells have been previously shown to contribute to ALS progression. In addition, recent studies have demonstrated that the motor neurons of ALS mice lack major histocompatibility complex 1 (MHCI) expression; this is important as MHCI expression protects cells from NK cell-mediated death. Unlike anti-inflammatory cell types that are currently being studied for use in ALS therapeutics, such as regulatory T cells, NK are purely pro-inflammatory and can be targeted in the blood using methods of depletion; thus, the purpose of this proposal is to determine the role of NK cells in driving ALS, with the long-term goal of developing future therapeutics. We hypothesize that NK cells contribute to ALS pathology by accelerating motor neuron death and by producing pro-inflammatory cytokines such as interferon-gamma, tumor necrosis factor-?, and granulocyte macrophage colony-stimulating factor that can skew downstream immune responses in the CNS. We therefore propose two Specific Aims which will assess the role of NK cells in both mouse and man. In the first Aim, we will deplete NK cells in ALS mice using commercially available antibodies to determine whether the absence of NK cells delays disease onset and slows progression. In the second Aim we will isolate NK and innate lymphoid cells from the blood of ALS patients and show using flow cytometry and qPCR that these cells are more prone to inflammatory cytokine production. We will also analyze the kinetics of NK cell accumulation and cytokine production by taking monthly blood draws from a cohort of ALS patients. These data will then be matched to clinical metrics of disease progression to demonstrate correlation between NK cells and ALS. Overall, completion of these studies will have considerable impact on both our understanding of the role of these cells in ALS pathogenesis and their potential as a therapeutic target.